The Ishiyama Hongan-ji War(石山合戦,Ishiyama Kassen?), taking place from 1570 to 1580 in Sengoku periodJapan, was a ten-year campaign by lord Oda Nobunaga against a network of fortifications, temples, and communities belonging to the Ikkō-ikki, a powerful faction of religious zealots. It centered on attempts to take down the Ikki's central base, the cathedral fortress of Ishiyama Hongan-ji, in what is today the city of Osaka. While Nobunaga and his allies led attacks on Ikki communities and fortifications in the nearby provinces, weakening the Hongan-ji's support structure, elements of his army remained camped outside the Hongan-ji, blocking supplies to the fortress and serving as scouts.

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The Ikkō-ikki, leagues of warrior monks and commoners were among the last to stand in the way of Oda Nobunaga's bid to conquer all of Japan. Oda had fought the Ikki before, crushing their armies of Mikawa Province and other areas, and by 1570, their twin fortresses of Ishiyama Hongan-ji and Nagashima were their last bastions of strength. He besieged both fortresses simultaneously, attacking Ishiyama in August 1570 and Nagashima in 1571.

In August 1570, Oda Nobunaga left his castle in Gifu with 30,000 troops, and ordered the building of fortresses around Ishiyama. On September 12, the Ikkō-ikki launched a midnight stealth attack against Nobunga's Kawaguchi and Takadono. The Ikko were reinforced by soldier monks from Negoro in Kii and 3,000 musketeers, pushing Oda's army back.[1][2][3]

While Oda himself focused on the sieges of the Nagashima fortress and other campaigns, his armies remained camped out, assigned to monitor the Ikki's fortress, and take it if they could.

After reducing the threat from the Ikki's supporters, Oda attempted to starve out the fortress. This was no easy task, however, because the Ishiyama fortress sat on the coast, which was guarded by the fleet of the Mōri clan, masters of naval combat and Oda's enemies. As early as 1575, however, the fortress was already in urgent need of supplies, and the Abbot Kōsa was ready to begin peaceful overtures with Nobunaga to end the siege. But the ousted Shogun, Ashikaga Yoshiaki, sent a letter to Mōri Terumoto asking for his aid in supplying the cathedral fortress.

In April 1576, Oda's army attacked with 3,000 men under the command of Akechi Mitsuhide and Araki Murashige. By then, 51 outposts had been built around the central fortress, many equipped with arquebus squads. The attackers were quickly repelled by 15,000 defenders. Oda Nobunaga was forced to revise his tactics and began to attack the outposts, and the supporters of the Ikki. He sent Toyotomi Hideyoshi to head an assault on the monk's fortress at Negoro-ji.

Nobunaga enlisted Kuki Yoshitaka to set up a blockade and disrupt the fortress' supply lines. In August 1576, in the first Battle of Kizugawaguchi, the blockade failed. But Kuki Yoshitaka returned two years later with massive new battleships and, in the second Battle of Kizugawaguchi, he broke the Mōri supply lines for good.

By then, the siege was beginning to swing in Nobunaga's favor. The majority of the Ikki's allies were already inside the fortress with them, so they had no one to call on for aid. Uesugi Kenshin, one of Nobunaga's strongest enemies and, therefore, among the Ikki's strongest allies, died in 1578, and the Mori clan lost their castle at Miki in 1580. Between 1578 and 1580, the Ikki sallied forth from the fortress many times under the leadership of Shimozuma Nakayuki to face Oda's armies. But by April 1580, the defenders were nearly out of ammunition and food. The Abbot Kōsa held a conference with his colleagues and, shortly afterwards, received a message from the Emperor himself asking that the Ikki surrender. It was no secret that this letter had been prompted by Nobunaga, but that would not change the power of an Imperial request; Kōsa surrendered several weeks later and fighting finally ended a few months after that, in August 1580.

Nobunaga spared the lives of many of the defenders, including Shimozuma Nakayuki, but burned the fortress to the ground. Three years later, Toyotomi Hideyoshi would begin construction on the same site, building Osaka Castle, a replica of which was constructed in the 20th century.